1. Field of the Invention
This invention relates to a heat treating apparatus and more particularly to the improvement of a heat treatment apparatus in which process gas is introduced into a reaction tube, with the reaction tube receiving articles to be heat-treated therein.
2. Description of the Related Art
In the conventional manufacturing process for semiconductor wafers a heat treating apparatus is used in which articles such as semiconductor wafers to be treated are placed in a reaction tube and are heat-treated by means of a CVD process, oxidation and diffusion.
The reaction tube of the heat treating apparatus is made of quartz (SiO.sub.2) and formed integrally. A gas introducing pipe and a gas exhausting pipe are provided at the lower portion of the reaction tube. The gas introducing pipe is adapted to cause the exterior of the reaction tube to communicate with the interior thereof. The forward end of the gas introducing pipe extends to the upper position of the interior of the reaction tube.
The gas exhausting pipe projects from the lower position of the reaction pipe.
On a boat provided in the gas exhausting pipe is disposed semiconductor wafers piled up at predetermined intervals. The boat carrying the semiconductor wafers is provided on a warming tubular member (heat holding member) which is raised and lowered by means of a lifting mechanism so that the semiconductor wafers are received in the reaction tube and are taken out therefrom.
When the warming tubular member is raised by means of the lifting mechanism and the semiconductor wafers are fully received in the reaction tube, the reaction tube is completely closed.
Heating means consisting of a heat generator and a heat insulating element is provided in the external region of the reaction tube. The heat generator is attached to the inner peripheral wall of the heat insulating element.
Process gas is introduced from the gas introducing pipe into the reaction tube, and the interior of the reaction tube is heated by means of the heat generator to a predetermined temperature (1,200.degree. C., for example). The semiconductor wafers are heat-treated due to the chemical reaction of the process gas which is being heated.
The reaction tube of this conventional heat treating apparatus is made of SiO.sub.2 from which few impurities are discharged during the heat treatment.
However, the reaction tube made of SiO.sub.2 encounters the problem in that it is likely to be distorted when it is heated at a high temperature (about 1,200.degree. C., for example) for a long time. In place of SiO.sub.2, silicon carbide (SiC) which has a higher thermal resistivity or, in other words, is resistant to the effects of heat, and is thus little distorted at a high temperature might be used for manufacturing a reaction tube in order to suppress the thermal distortion of the reaction tube.
Since, however, the machinability of SiC is poor, it is difficult to manufacture relatively complicated parts of a reaction tube, such as a gas introducing pipe and a gas exhausting pipe, with SiC. Further, the thermal conductivity of SiC is more than ten times larger than that of SiO.sub.2. Thus, when the entire reaction tube is made of SiC, a large amount heat is radiated from the lower region of the reaction tube which is not covered with a heat insulator. The portion in which semiconductors are loaded is affected by the heat radiation and the uniform thermal distribution in the region is disturbed.
In the conventional heat treatment process, a reacting substance is likely to become attached to the gas introducing pipe. In order to perform the heat treatment in a good condition, therefore, the gas introducing pipe must be washed and the attached reacting substance must be removed therefrom. The gas introducing pipe in the reaction pipe must be changed according to the kind of heat treatment process which is selected from various kinds of heat treatment processes. Since, however, the conventional gas introducing pipe is integrally formed with the reaction tube, the tube cannot be washed or changed easily.